In recent years, a photocatalyst such as titanium oxide has been used in many use applications including an exterior material of an architectural structure. A photocatalyst excited by a light can decompose various harmful substances, and utilizing this property allows a surface of a base coated with a photocatalyst to be cleaned. In addition, a photocatalyst which is excited by a light makes a surface of a base coated with the photocatalyst hydrophilic, and the hydrophilic properties thus imparted allow a dirt which is attached to the surface to be easily washed out by water. It is widely known that the photocatalyst coated film like this is formed by applying a coating solution which contains a photocatalyst. Besides, a photocatalyst coated film mainly used is transparent so as not to damage the design of a base such as an exterior material.
A technology in which hydrophilicity is given to a surface of a glass, a mirror, or the like by using, as a coating solution which contains a photocatalyst, an aqueous dispersion which contains photocatalyst particles and an inorganic binder has been known (see, for example, PTL 1: JP 2001-89706 A). In the technology described in PTL 1, a transparent coated film is obtained by highly dispersing fine photocatalyst particles and an inorganic binder such as an alkaline silicate salt. PTL 1 also discloses that in order to suppress uneven coating of the coating solution so that the coated film may have a uniform film thickness, the coating solution is impregnated into an unwoven cloth and is applied to a base by sliding it to one direction at a constant speed. Namely, in this technology, a worker involved in application has been required to have a high skill.
In order to form a satisfactory photocatalyst coated film without requiring a high skill to a worker, various ingenuities have been exercised. For example, as to an apparatus or a tool for assisting the application work, in JP 2003-026447 A (PTL 2), a method in which application is conducted while a roller is in the fixed state is proposed; and in JP 2010-247054 A (PTL 3), it is proposed that a laser irradiation means and a metronome are combined with a spraying apparatus.
Also, with regard to improvement of a composition of a coating solution focusing on rheology, for example, it is proposed in PTL 1 that a surfactant or a thickener is added to a dispersion in order to control viscosity of the dispersion. It is proposed in JP 2004-143443 A (PTL 4) that a thickener is used and that a surfactant and a solvent having a high boiling point are added.
However, even with these technologies, the status quo is that a high skill is required to a worker involved in application especially when a coating solution is applied on the spot to a large area such as a wall surface. In order to form a satisfactory photocatalyst coated film without requiring the special skill as mentioned above, further improvement of the coating solution is necessary.
It is proposed that, in WO 2000-33977 A (PTL 5), an organic colorant is added to a photocatalytic coating composition to improve visibility of the composition so that a formation of a photocatalyst coated film may be confirmed. According to the PTL 5, it is described that the color of the organic colorant disappears after application by photocatalytic action.